Soil Chemical Analysis as a Tool for Evaluating Nutrient Availability

Soil test results may be very confusing and raise many questions, such as  – Why do different labs give different results for the same soil sample? Why do they give a different interpretation? Why can’t we just test the actual amount of nutrients in the soil? What is the meaning of “available nutrients”? Why are there so many different testing methods? And more…

The soil testing process consists of four steps:

• Soil sampling;
• Chemical and physical analysis;
• Correlation and calibration of results;
• Fertilizer recommendation philosophy.

This article will focus on the chemical analysis step.

THE MEANING OF “NUTRIENT AVAILABILITY”

Many different elements in soil are potentially available to plants, and the purpose of soil testing is to estimate them as accurately as possible.

Soil consists of three phases which exist in equilibrium – solid phase, liquid phase and gas phase. The most important equilibrium exists between the soil phase and the liquid phase (soil solution). Plants absorb nutrients from the soil solution. However, the amounts of nutrients in the soil solution are insufficient to sustain plant growth.

When nutrients are depleted in the soil solution, they are replenished from the solid phase of the soil. This occurs through reactions such as dissolution, desorption, oxidation, reduction, hydrolysis or microbial mineralization reactions.

Testing the soil solution alone will underestimate the amount of nutrients really available to the plant, while measuring the total amount of nutrients present in the soil will result in an overestimation of the nutrients available. This is because a large portion of them is strongly bound to soil particles or exists in a form which is not available to plants.

Therefore, to determine the elements that are available for plants in soil, the soil test must quantify a relationship between the solid phase and the soil solution.

WHY IS IT DIFFICULT TO ESTIMATE THE AVAILABLE NUTRIENT LEVELS?

Soil chemical tests are mostly empirical from several reasons:

• In the sampling and testing procedure, the soil loses its physical structure, and soil particles are in close contact with the chemical used for the extraction of the nutrients. This close contact optimizes desorption/dissolution reactions, and therefore may result in overestimate or underestimate of the amount of nutrients that can reach the plant roots.
• The physical accessibility of nutrients to plant roots affects their availability to plants.Immobile nutrients, such as P and Zn, are less available, thus the quantity of nutrient extracted usually differs from the quantity actually available to the plants.
• Plants vary in their capability to absorb nutrients, as they have different root architecture and activity.
• Other environmental factors, such as soil water status affect both root growth and nutrient availability.
• Soil properties, such as pH, mineral composition and nutrient buffering capacity also affect the capacity of the soil to release nutrients into the soil solution.

PRINCIPLES OF INTERPRETING SOIL TEST RESULTS – Why do Different Labs Give Different Results and Interpretation?

1. Analytical Methods

Different chemicals and testing methods extract different quantities of nutrient from soil.

Therefore, different testing methods are used for different soil properties and conditions. The success or failure of any chemical soil test greatly depends on the mode of action of the extracting chemical, or the extraction method used. The last two must simulate or correlate with plant uptake for a particular soil.

Therefore, different laboratories may give different results for the same sample.

In most cases, even when the numerical results are different, due to the use of different analytical methods, the interpretation is mostly the same in terms of sufficiency levels of the nutrients, i.e. deficient, adequate, high etc.

In case one lab uses testing methods which are not adequate for the specific soil and conditions, the interpretation itself might differ.

For example, at a given soil test level the interpretation may be optimum for one test, but may be interpreted as low or high for another test. This may be very confusing.

2. Procedures

Differences in laboratory procedures, such as different dilution, shaking rates, extraction time, filter paper etc. may also affect the results and lead to differences in the soil test results, even if the laboratories use the same testing methods.

Before sending soil samples to the lab, make sure that the lab you choose uses the appropriate  testing methods for your soil.

For example, two common testing methods for phosphorus are Bray-1 (uses 0.025M HCl and 0.03M NH4F) and Olsen (uses 0.5M sodium bicarbonate). The Bray method extracts more phosphorus than Olsen and is more adequate for soils with pH<7.0, while Olsen is more adequate for calcareous soils.

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